1. Field of the Invention
The present disclosure relates to a catalyst support useful for preparing a catalyst that serves to oxidize carbon monoxide (CO), particularly, the catalyst useful for oxidizing carbon monoxide preferentially in hydrogen-rich gas produced by the reformer, which is required by proton exchange membrane fuel cell (PEMFC).
2. Description of Related Art
Catalysts for oxidizing CO are well known in the prior art. Such catalysts have utility in a number of fields including the treatment of exhaust gas streams from internal combustion engines, such as automobiles and motorcycles, and the treatment of hydrogen-rich gas produced from the reformer required by PEMFC. Among those catalysts, a so-called “PROX” (preferential oxidation) catalyst is created specifically for the oxidization of CO in hydrogen-rich gas owing to its selective removal of CO.
In a fuel cell, hydrogen is often used as the fuel and supplied to the fuel cell's anode. Oxygen, on the other hand, is the cell's oxidant and is supplied to the cell's cathode. The hydrogen used in the fuel cell usually can be derived from the reforming of methanol, alcohol, methane, liquid petroleum gas (LPG), or other organics. For example, in the methanol reformation process, methanol and water are reacted to generate hydrogen and carbon dioxide according to the reaction: CH3OH+H2O→CO2+3H2. Unfortunately, the effluent exiting the reformer contains undesirably high concentration of CO which can quickly damage the fuel cell's anode, and accordingly must be removed.
It is known that CO level in the hydrogen-rich gas exiting a reformer can be reduced by performing a so-called “water-gas shift” (WGS) reaction. In the WGS reactor, water is added into the effluent of the reformer (in the presence of a suitable catalyst) to lower its temperature, and at the same time, increase the steam to carbon ratio therein. The higher steam to carbon ratio serves to lower the CO content in the effluent according to the reaction: CO+H2O→CO2+H2. Nevertheless, the WGS reaction is still not sufficient to reduce the CO content in the effluent to an acceptable level (below ˜30 ppm). Therefore, it is necessary to further remove the CO content in the effluent exiting the WGS reactor before reaching the fuel cell.
It is known that PROX reaction in a suitable PROX reactor can further reduce the CO content in the effluent exiting the WGS reactor according to the reaction: CO+½O2→CO2. The PROX reactor comprises a PROX catalyst operated at temperature which promotes the preferential oxidation of CO by air in the presence of hydrogen, but without consuming substantial quantities of hydrogen.
Typically, prior art catalysts comprise a precious metal component such as platinum, palladium, rhodium, copper, etc., deposited on a metal oxide support such as alumina, silica, titania, zeolite or the combinations comprising the foregoing. Although precious metal component of the catalyst is crucial in determining its effectiveness, catalyst support material also plays a major role in affecting its performance as well as its application. Its surface properties, such as the specific surface area, the surface acidity, thermal stability, the average pore size and the pore size distribution, etc., have decisive influence on the performance of the catalysts.
U.S. Pat. No. 4,134,860 relates to the manufacture of catalyst. The catalyst composition can contain platinum group metals, base metals, rare earth metals and refractory, such as alumina support.
While alumina is the typical choice of catalyst support owning to its high temperature stability, one of its deficiencies is that it is vulnerable to sulfur-poisoning in the exhaust gas stream. That is, the alumina reacts with oxides of sulfur like SO2 and SO3 to form a sulfate Al2(SO4)3 which is stable at high temperature. Formation of alumina sulfate results in a decrease in the surface area and pore size of the alumina. Hence, lowering the efficiency of the catalyst since less catalyst is exposed to the exhaust gases. It is known that titania is resistant to SOx poisoning at the high operating temperature in the automotive exhaust gas system. U.S. Pat. No. 5,922,294 discloses a catalyst support containing titania which can be used in making the catalyst for treating vehicular exhaust gases.
Another approach to improve the performance of the alumina support is to introduce silica into the alumina support. U.S. Pat. No. 4,134,856 discloses a process for preparing alumina support containing silica by co-precipitation.
U.S. Pat. No. 6,235,255 relates to a platinum metal catalyst dispersed on a catalyst support comprising zeolite. The catalyst made from such support is used for the treatment of exhaust gases, such as carbon monoxide, from “lean-burn” engines.
U.S. Pat. No. 6,780,805 discloses a zeolite/alumina composite catalyst support which exhibit high surface area. The metal catalyst platinum/rhodium deposited on the composite support is used for the conversion of vehicular exhaust gases including carbon monoxide.
A CO preferential oxidation catalyst comprising CuO deposited on CexZr1-xO2 carrier has been demonstrated to show excellent CO removal efficiency as well as good CO selectivity (Chen et al., International Journal of Hydrogen Energy, 2006, 31, 427-435). However, the application of such catalyst is limited by its powder form.
Up to now, various catalyst support materials have been utilized in making an effective catalyst for the treatment of exhaust gases, particularly carbon monoxide, produced by the internal combustion engines or the reforming reaction to generate hydrogen-rich gas for PEMFC. However, for both practical and commercial applications, the effectiveness of the catalyst in treating the gases is not the only concern. Besides good performance, it is also essential to come up with a supported catalyst that is low-cost, durable, easy to handle (inert), and widely applicable.